The invention relates to a reforming reactor for the water vapor reforming of methanol, such as may be used, for example, to produce hydrogen in fuel-cell-operated vehicles.
U.S. Pat. No. 5,248,566 describes a reforming reactor suitable for use in fuel-cell-operated motor vehicles. It has three serially arranged reactors stages, each of which is charged with a suitable reforming catalyst material. The inlet-side reactor stage is charged with a suitable catalyst material for the partial oxidation of methanol, while the center reactor stage is charged with another suitable catalyst material for water vapor reforming of methanol. The outlet-side reactor stage is designed to implement a conversion reaction from carbon monoxide to carbon dioxide. The heat required for carrying out the endothermal water vapor reforming reaction in the center reactor stage is generated by the exothermal partial methanol oxidation in the inlet-side reactor stage, and is transferred to the center reactor stage by the flow of reaction gas.
U.S. Pat. No. 5,401,589 discloses a three-stage reforming reactor for water vapor reforming of methanol, which includes a reaction space filled with a catalyst pellet fill, and is divided into three reactor stages which are situated below one another and are held at different temperatures. The inlet-side stage is maintained at approximately 300.degree. C.; the center stage is maintained at approximately 275.degree. C. and the outlet-side stage is maintained at approximately 225.degree. C. The heating of the three reactor stages takes place by way of a heat-conducting dividing wall through which these reactor stages with different contact surfaces are in a thermal contact with a common heating space.
U.S. Pat. No. 3,522,019 discloses a reforming reactor of the initially mentioned type in which three separate reaction spaces are filled with a catalyst material, with an external heating device in the form of a burner being assigned only to the center stage. The inlet-side reactor stage can be heated via an internal heat exchanger and the hot reaction gas which comes out of the center stage, and for this purpose is guided in a counterflow through the heat exchanger. The heat generated in the third reactor stage is coupled into a connected hydrogen purification unit.
The object of the invention to provide a new reforming reactor of the initially mentioned type, which can be implemented at relatively low expenditures and is particularly suitable for mobile applications.
This and other objects and advantages are achieved by the reactor according to the invention, in which the inlet-side reactor stage, which is supplied with heat from the outlet-side reactor stage, can convert a small portion of the hot entering gas mixture, reducing the concentration of the gas constituents that are to be reformed, at the start of the center reactor stage. This permits a smaller construction of the center stage and its slower aging as the result of the gas constituents to be reacted. At the same time, as a result of its catalyst material charge (preferably in the form of a catalyst pellet fill), the inlet side reactor stage is capable of filtering out floating particles, so that a hot gas filter in front of the center reactor stage becomes unnecessary. In addition, water and methanol drops can be filtered out by the inlet-side reactor stage, preventing resultant damage to the catalyst material in the center reactor stage, which is important for the implementation of the reforming reaction. It is unnecessary, therefore, for the educt gas flow to be free from drops for this purpose; this feature, in turn, permits a simpler construction of an evaporator connected in front of the first reactor stage. By means of a corresponding heating, the main reforming reaction takes place in the center reactor stage in the temperature range optimal for this purpose.
Because it is unnecessary to heat the third reactor stage, a conversion reaction can take place here, by which the carbon monoxide is converted to carbon dioxide, so that the carbon monoxide fraction in the reaction gas can be reduced. Moreover, because of the heating capacity of the outlet-side reactor stage, temperature fluctuations in the center reactor stage, such as occur during load changes, are damped so that, while the construction is small, the center reactor stage can be operated at higher temperatures and larger temperature fluctuations. In addition to fluctuations of the gas outlet temperature, as a result of the outlet-side reactor stage, fluctuations of the carbon monoxide content in the reaction gas during load changes are also reduced. By means of the thermal coupling of the inlet-side with the outlet-side reactor stage, a self-regulating heat transmission mechanism is implemented during load changes, in which case the lower inlet and outlet temperatures of the gas mixture permit a higher efficiency.
The inlet-side and the outlet-side reactor stages may be connected with one another in a heat exchange connection by way of a heat conducting dividing wall. Alternatively, they are spatially separated from one another and heat exchange is performed by the reaction gas flow emerging from the outlet-side reactor stage, which is guided through a tempering space that is in thermal contact with the inlet-side reactor stage.
In one embodiment of the invention, the inlet and outlet side reactor stages together form an independent heat exchanger unit which is spatially separated from the center reactor stage. Thus, for example, a single-stage reforming reactor can be upgraded to a three-stage reforming reactor by means of a simple retrofitting with such a heat exchanger unit.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.